Proceedings of the 36th European Microwave Conference

A Compact Quasi-Elliptic Dual-Mode Cavity Filter Using LTCCTechnology for V-band WLAN Gigabit Wireless Systems

J.-H.Lee(l', N. Kidera(2), S. Pinel(l), J. Laskar(l) and M.M.Tentzeris(l)

(')School ofECE, Georgia Institute of Technology, Atlanta, GA 30332-250, U.S.A.(2)Asahi Glass Co., Ltd. 1150 Hazawa-cho, Kanagawa-ku, Yokohama, Kanagawa 221-8755, Japan

e-mail: jonglee(ece.gatech. edu

Abstract - In this paper, the development of highlyintegrated dual-mode filter solutions is presented for V-bandcompact, low-cost wireless front-end modules utilizingmultilayer low temperature cofired ceramic (LTCC)technologies. A dual-mode cavity filter experimentallydemonstrates a quasi-elliptic response and excellentperformance in terms of low insertion loss (<2.76 dB) and highrejection. The rectangular shaped cavity resonator is firstdesigned to generate two resonant modes (TE102 and TE201) thatare orthogonal each other. Then, the location of the feedingstructures and the size of the external slots are optimized togenerate the quasi-elliptic dual-mode response. The filter hasbeen implemented by employing rows of vias as sidewallsrepresent a new class of devices that enable the threedimensional (3D) integration for V-band WLAN gigabit wirelesssystems.Index Terms - dual-mode filter, V-band, gigabit wireless

systems, cavity filter, LTCC.

I. INTRODUCTION

The rapid expansion of wireless communications and personalcommunication networks has led to tremendous demands ofminiaturization, portability, low-manufacturing cost and highperformance in RF and millimeter-wave (mmW) wirelesssystems [1]. Especially, the appeal for transmitting multimediainformation using high-speed digital data and wide-band imagesignals has motivated the development of 60 GHz wirelesscommunication systems because of their high potential foractualizing compactness and wide bandwidth [2]. The 60 GHzfront-end module is the foundation of these systems andrequires the high performance filters that can be easilyintegrated into the modules.

This paper focuses on the development of 61.6 GHz quasi-elliptic dual-mode cavity filters enabling low loss filtersolution for RF Front-end module of V-band WLAN gigabitwireless systems. The rectangular shaped cavity resonator isfirst designed to generate two resonant modes (TE102 andTE201) that are orthogonal each other. Then, the externalcoupling to resonant modes is employed to providetransmissions zeros that achieves the high selectivity. Themultilayer LTCC process is utilized for the fabricationbecause of its mature packaging capability of integratingembedded functions. The performance of the proposed

structure is experimentally validated for the first time at thesehigh frequency ranges.

II. SINGLE DUAL-MODE CAVITY BAND PASS FILTER (BPF)

As the demand for compact and low loss band pass filtersincreases in V-band multi-gigabit-per-second wirelesscommunication systems, integrating on-package cavity filtersin LTCC multilayer technology are a very attractive optiondue to their relatively high Q and power handling capabilitycompared to planar filter structures and less interference fromother circuits utilized in packaging [3]. A conventionalcircular or rectangular waveguide dual-mode filter makes useof the coupling of two orthogonal modes generated fromtuning screws [4] or rectangular ridges [5]. However, thesetechniques not only impose the heavy numerical burden forthe modal characterization of waveguides because of a largenumber of evanescent modes but also are not applicable toLTCC multilayer process. A microstrip-fed dual-mode filterbased on the substrate integrated waveguide (SIW) technique[6] also has been proposed to be one of possible passivesolutions for L-band millimeter-wave systems, developedwith a relative thick substrate and a low dielectric constantcompared to LTCC tape.

In this paper, we propose a 61.6 GHz (V-band) quasi-elliptic dual-mode filter utilizing two arrays of via fences assidewalls that can be easily integrated into a wireless RFfront-end module in LTCC. The very mature multilayercapability of LTCC (Fr=7.1, tan6=0.0019) has been used tofabricate the proposed structure. The cavity filter is embeddedinto the first three substrate layers (four metal layers) of theLTCC substrate and consists of one cavity resonator, twomicrostrip lines for input/output (1/0) feeding structures andtwo external coupling slots etched on the top ground planes ofthe cavity.

Figure 1 shows (a) the overview and (b) the top view of theproposed structure. The rectangular shaped cavity resonatoris first designed to generate two resonant modes (TE102 andTE201) that are orthogonal each other, realizing dual-modeoperation. The effective length, width (L, W in Fig. 1 (b),respectively) of the cavity resonator based on TE102 and TE201modes can be determined by constraining both modes toresonate at the same frequency in the resonant frequencyequation of the rectangular waveguide cavity [7]. The heightof the cavity is determined to be 0.106 mm to satisfy the

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5| 1472e+005q. 7795e+005q.q1i9e+005c: cq.Oqq~2e+005

3.6766e+0452, 9413e+0052, 5736e+0052. 2059e+0051, 8383e+005iA706e+005

I i,1030e+0057.3532e+E1f00 D C U (2 ()Q3, 6766e+00q~9.039Se-00q 00

Figure 2. Magnitude of electric field distribution (TE102) onhorizontal plane inside the dual-mode cavity at the resonantfrequency (=61.6 GHz).

TABLE I3-POLE CAVITY BPF DIMENSIONS

Figure 1. (a) The overview (b) The top view of the quasi-ellipticdual-mode cavity filter.

compactness requirement. However, the height can beadjusted to obtain the desired quality factor (Q). The dual-mode cavity resonator is built using conducting planes ashorizontal walls and via fences as vertical walls [3] based onthe dimensions calculated, and the final dimensions areoptimized with the aid of the full-wave simulator HFSS. Twoexternal slots (Fig. 1 (a)) on the top ground plane of the cavityare dedicated to magnetically couple the energy from the 1/0microstrip lines into the cavity resonators. The microstriplines are terminated with shorting vias to avoid theinterference coupling between the feeding structures [8]. Theoffset distance (D in Fig. 1 (b)) of the feeding structures is thecritical design parameter to excite two resonant modes (TB102and TE,01) and to generate the transmission zeros for the highselectivity. The transmission zeros that realize the quasi-elliptic response are provided by two factors: 1) the externalcoupling to two resonant modes that have the same amplitudebut anti-phase at the I/O apertures 2) the parasitic couplingfrom the external coupling slots. Also, the position of feedingstructures affects the coupling level to the modes, thusinfluencing the bandwidth. The size of the external couplingslots (EL xEw in Fig. 1 (b)) is another important designparameter to control the coupling between the feedingnetwork and the cavity and the positions of the transmissionzeros that determine the rejection level. All the layoutdimensions are optimized in the HESS full-wave simulatorand summarized in Table I.

Design Parameters Dimensions (mm)cavity length (L) 2.075cavity width (W) 2.105cavity height (H) 0.106

external slot length (EL) 0.360external slot width (EW) 0.572external slot position(ED) 0.3 52

Offset distance of the feedlines (D) 0.568

Figure 2 shows the electric field distribution of TE102 modeat 6.16 GHz that is bisected along the diagonal line. Theelectric fields in two sections represent the same magnitudebut with 180° phase differences. Figure 3 shows thecomparison between the simulated and the measured S-parameters of the dual-mode cavity filter. All fabricatedresonators were measured using the Agilent 8510C NetworkAnalyzer and Cascade Microtech probe station with 250 vmpitch air coplanar probes. A TRL method was employed forcalibration. The filter exhibits an insertion loss <2.76 dB,which is slightly higher than the simulated (2.22 dB). Themain source of this discrepancy might be caused by skin andedge effect of metal traces since the simulations assume aperfect definition of metal strips. The center frequency ismeasured to be 61.6 GHz that exactly matches the simulatedresult. The transmission zero is observed within < 3.5 GHzaway from the center frequency at the right-hand side of thepassband and < 6.75 GHz at the left-hand side. Thediscrepancy of the zero positions between the measurementand the simulation can be attributed to the fabricationtolerance. The tolerance of XY shrinkage of the LTCCprocess is expected to be - 3 % and that of metal dimensionalaccuracy ±0.1%. The fabrication tolerance also results in the

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54 56 58 60 62

Frequency (GHz)

Figure. 3. Measured and simulated S-parameters ofcavity BPF.

bandwidth differences that the filter exhbandwidth about 4.13 % (- 2.5 GHz) conrsimulated 2 % (. 1.25 GHz).

III. CONCLUSION

We have presented the development ofelliptic dual-mode cavity filters that can be hlinto LTCC RF front-end modules for V-bandwireless systems. For the first time at theseranges, this type of filters has experimentally C

excellent performance in terms of low inserticrejection. The quasi-elliptic dual-mode responfilter has been realized by adjusting the cavity

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slots positions and can be improved by cascading or stackingthe dual-mode cavities with inter-coupling structures.

ACKNOWLEDGEMENT

The authors wish to acknowledge the support of the GEDCDesign Center, the NSF Career award, and the NSF Grant#ECS-03 1395 1.

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